1. Field of the Invention
The present invention relates to the formation of polymeric optical waveguides, or more particularly to a process for forming single-mode, organic waveguides employing organic polymeric materials. The process reduces dissolved and gaseous oxygen content to very low quantities, resulting in production of waveguides having superior properties and manufacturability.
2. Technical Background
In optical communication systems, messages are transmitted by carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems. They have a greatly increased number of channels of communication as well as the ability to transmit messages at much higher speeds than electronic systems using copper wires. This invention is concerned with the formation of light transmissive optical waveguide devices. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
It is possible to produce polymeric optical waveguides and other optical interconnect devices which transport optical signals in optical circuitry or optical fiber networks. One method used to form an optical device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical interconnect applications because they can be patterned by photolithographic techniques which are well known in the art. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer containing layer deposited on a substrate.
Planar polymer waveguides typically comprise layers of low loss optical materials of precise indices of refraction. Both step index and gradient index waveguide structures are known in the art. For planar polymer and glass waveguides, in particular, step index structures are most easily achieved through successive coating of materials with differing indices of refraction. Typically, a waveguide core has a refractive index which is 0.3% to 2% higher than a clad. The magnitude of this refractive index difference (Δn) is set to optimize the performance of the planar waveguides or to match light modes when the transition is made from the planar device to an optical fiber.
Waveguides can be made using photopolymerizable optical materials which can be coated and cured on a substrate. Typically, the materials include mixtures of monomeric and oligomeric components which are blended to provide the correct index of refraction. Among the many known photopolymers, acrylate materials have been widely used as waveguide materials because of their optical clarity, low birefringence and the ready availability of a wide range of monomers. These photopolymerizable materials, especially acrylate type materials, have properties that make their processing very difficult. In particular, dissolved and gaseous oxygen present within or in the proximity of the photopolymerizable material quenches polymerization and therefore its abundance must be carefully regulated both within and at the immediate surface of the material. The presence of oxygen affects the speed of cure, the degree of cure, the degree of resolution, and the minimum feature size which can be created by a lithographic patterning process. Also, since some of these photopolymerizable materials remain a low viscosity liquid at room temperature, they require handling in a manner which does not disturb the thin film which is to be lithographically imaged. For this reason some conventional means of excluding oxygen, such as the use of a direct-contact UV-transparent cover, cannot be readily employed.
Typical acrylate curing processes are capable of reducing or eliminating the effects of oxygen by either reducing the amount of available oxygen or overcoming its presence through the use of high UV power flux. One typical process is to introduce a solid barrier in contact with the film. This barrier is sufficient to allow a strong UV light source to polymerize the liquid system without residual oxygen hindering the polymerization process. This may be done by pressing a sheet with a low oxygen permeability against a film composition for the purpose of reducing the amount of oxygen in the composition. In another process, a PET film is applied to a liquid monomer film. However, residual oxygen within the photopolymerizable material can surprisingly degrade the fine lithography needed for formation of fine lithographic structures, e.g. single mode waveguides. In addition, the placement of a solid surface upon liquid photopolymerizable material leads to film non-uniformities due to surface tension of the liquid film interacting with the solid substrate below and above. These film non-uniformities degrade the required lithographic structures required, e.g. for single-mode waveguides. Even with a liquid photopolymerizable material with a very high viscosity that prevents flowing and the creation of long range film non-uniformities, the placement of an oxygen barrier substrate against this surface can introduce particulate contamination, can stick to the film and create defects due to adhesion of the barrier plate to the film, and will not conform perfectly to this drier film, thus allowing intrusion of oxygen during the polymerization process.
Another typical process is to reduce the oxygen concentration in the material and its environment to about 2%. This reduction in oxygen concentration is sufficient to allow a strong UV light source to polymerize the liquid system without the remaining oxygen hindering polymerization. However again, residual oxygen within the photopolymerizable material along with oxygen replenishment from residual oxygen left in the purge ambient can markedly degrade the reproducibility of the cure of the material and can surprisingly degrade the fine lithography needed for formation of fine lithographic structures, e.g. single-mode waveguides. Also, different materials will have different sensitivity to residual oxygen and can prevent curing. A less typical process involves application of a non-polymerizing, inert liquid barrier layer. Layers such as water, glycerin, or ethylene glycol, will act as reasonable barrier layers, however, this process has several deficiencies. The layer must be applied without mixing with or dewetting from the lower layer, it must be removed without contaminating the photopolymerizing lower layer, and finally it requires that a photomask used for imagewise exposing the photosensitive composition be further removed from the photopolymerizing layer, thus reducing the lithographic resolution.